This invention relates to a process and an apparatus for the production of soluble instant coffee in powder form.
Soluble coffee powder is conventionally produced by freeze-drying or spray-drying after evaporation of a coffee extract obtained by the percolation of an extraction liquid through cells filled with ground roasted coffee (Sivetz, Coffee Processing Technology, Volume 1, pages 262, 263, AVI, 1963).
Extraction is carried out in countercurrent fashion, i.e., water under pressure at a temperature of 150.degree. to 180.degree. C. is introduced into the cell containing the batch of ground roasted coffee which has been most intensively extracted (having undergone N extractions) at the bottom of that cell, and then liquid extract of this extraction cell is passed through the extraction cell containing the batch of coffee which has been extracted (N-1) times, and so on, until the liquid extract passes through the cell which has just been filled with fresh ground roasted coffee, and the final extract leaves the last cell at a temperature on the order of 100.degree. C.
In such countercurrent extraction, the most intensively extracted coffee is thus subjected to the highest temperature while the fresh coffee is subjected to the lowest temperature.
A distinction is normally drawn between the hot cells, which contain the most intensively extracted coffee, and the cold cells, which contain the least intensively extracted coffee.
After each extraction cycle, the cell containing the most intensively extracted coffee is emptied, filled with fresh coffee and, after the cells have been suitably interconnected, another extraction cycle begins.
Although the final extract obtained at the exit of the extraction cell containing the freshest coffee contains only a small quantity of ground coffee particles, fines always being entrained, it is necessary to filter the extract.
Finally, after the filtration phase which eliminates the particles larger than about 1 mm in size, solids, such as polysaccharides or proteins, are still present in suspension and have to be eliminated to enable a coffee powder which dissolves perfectly without any solids appearing in the cup to be obtained after concentration and freeze-drying or spray-drying of the extract.
The suspended solids are normally eliminated by centrifugation, the sediment obtained then being decanted, the supernatant decantation liquid being reintroduced into the final filtered extract while the solid residue obtained is eliminated.
The main disadvantage of this process is that it produces a sediment which has to be retreated by decantation and which is not easy to handle.
In addition, the suspended solids cannot always be satisfactorily eliminated by centrifugation.
Accordingly, the problem addressed by the present invention was to provide a process for the production of instant soluble coffee powder in which extraction of the coffee in the liquid phase would enable the content of insoluble solids in the final extract to be reduced.